docs: Fix formatting for warning callouts

[yumemio]

In the Dockerfile reference (1, 2, 3), the warning callouts were not rendered as intended, as shown in the picture below. Note the background color, the "i" icon and the trailing class attribute ({:.warning}):

Here is how they should appear instead (content not relevant to the issue):

The PR updates the class attribute to match the Formatting Guide.

[yumemio]

Redoing the commit with sign-off. Please ignore this PR.